The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Radio frequency power can be expressed in the time domain as a periodic signal that has particular peak-to-peak (P-P) amplitudes at corresponding times. The P-P amplitudes describe the envelope of the RF power. Amplitude modulation (AM) is an example of a RF mode that modulates the RF envelope.
RF power can be used to generate plasma that can be used to fabricate semiconductors by using methods that are known in the art. In such applications it is increasingly important to precisely control the RF envelope as the size of semiconductor features decreases and/or as demands on production yield increase. Several methods are known in the art for controlling the RF envelope and are described below.
A first group of methods provide RF on-off envelope pulsing by switching a RF power amplifier driver on and off and/or using on-off control circuits of a power supply that feeds the RF power amplifier to pulse a DC input rail voltage to the power amplifier.
A second group of methods provide RF envelope pulsing by controlling the amplitude of the RF power amplifier driver. These methods are typically applicable to power amplifiers with Class A, AB, B and C circuit topologies and/or controlling the output voltage of a power supply that feeds the RF power amplifier to modulate the DC input rail voltage to the power amplifier.
A third method is disclosed in U.S. Pat. No. 7,259,622, which provides a phase-controlled RF power amplifier design with a full-bridge topology to facilitate flexible RF envelope waveform generation. The bridge topology requires four power switches and a complex gate drive control scheme. Those design features increase parts count and cost and reduces reliability.
The above methods also have some inherent operating limitations. In the methods that use the power supply control for RF envelope pulsing (on-off or multi-amplitude-level), the pulse rise and fall speeds are limited by the dynamic response of the DC rail voltage. RF pulse envelopes with fast rise and fall times, high pulsing frequencies, or pulses with small duty cycles are therefore not easily achievable. The above methods that use RF power amplifier drive on-off control will only facilitate RF envelope on-off pulsing. They are unable to provide an RF of varying non-zero amplitudes. The above methods that use the RF power amplifier drive control for RF multi-amplitude-level envelope pulsing are limited to linear power amplifiers (such as with Class A, AB, B & C topologies), and not easily extendable to high efficiency switch-mode power amplifiers (such as with Class E topology). Linear power amplifiers also require designs and constructions with large power handling and thermal capacities to dissipate heat because large differential powers need to be dissipated from the power amplifier transistor devices during pulsing.
With a combination of power supply control and RF power amplifier drive control, some limitations listed above can possibly be reduced. However, such reductions would be limited and require complex control circuits for RF envelope pulsing, as both the power supply control and the power amplifier drive control would need to be well coordinated while pulsing.